When implementing cellular telephone systems, it is important to maximize the amount of data that can be transmitted at the available frequency. This often involves altering the data signals to permit the transmission of more data. These alterations must be reversible so that the original signal can be sufficiently recovered at the receiving end. One method of altering the data is through code division multiple access (CDMA) and spreading.
Spreading is a method of altering data whereby a normally narrowband signal is translated into a very wide band signal. Spreading a signal over a wide frequency band allows the signal to be transmitted and received using less power. Spreading is achieved by multiplying the original signal by a code which expands and duplicates the signal. The number of times the signal is duplicated over the wide frequency band is known as the spreading factor. For example, a spreading code of (1, 1, 1, 1) has a spreading factor of 4 because each signal portion (known as a “symbol”) is repeated four times when the code is multiplied by a signal. The above example of a spreading code, when multiplied by a signal comprised of symbols “abcd”, yields a spread signal of “aaaabbbbccccdddd.” If the spreading code is (1, −1, 1, −1) issued to spread the same symbols then the spread signal is “aāaāb bb bc cc cd dd d” where x represents the negative of the original symbol x. Once spread, the wide band signal is transmitted to the receiver. The signal is despread at the receiver end to recover the original signal. The code used to spread the signal is also used to despread the signal.
Because of several factors, including the intensive computational requirements of a despread function, receivers have implemented despreaders which have included the use of ASICs and the use of a hardware block having the most basic components in a digital signal processor. These implementations suffer from lack of programmability. Other despreaders have included the use of FPGAs, but these are undesirable because of cost and because they do not offer sufficient flexibility in programming.
A digital signal processor, rather than an ASIC, is a desirable solution because of its software programmability. However, it is difficult for a digital signal processor to handle the high speed complex calculations necessary to implement a satisfactory despreader.
Accordingly, there is a need for improved implementations of despreaders for use in decoding transmitted wireless telephone signals.